In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water (0 °C or 32 °F) for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of the ground material. A pioneering airborne electromagnetic survey in the Yukon Flats near Fort Yukon, Alaska, by the U.S. Geological Survey has yielded unprecedented images of the presence and absence of permafrost to depths of roughly 328 feet. The airborne survey captured images of permafrost over a substantially larger area, and with greater data density, than has been previously achieved using sparse boreholes and ground-based geophysics.

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"Liquid water conducts electricity better than ice," explained USGS director Marcia McNutt. "We can detect from the air the weak magnetic fields generated by those electric currents, thus distinguishing quickly and easily melted from frozen ground. This new technology, and the maps of changing permafrost, will be valuable for both climate change research and engineering in the challenging Alaskan environment."

Because the Yukon Flats is near the boundary between continuous permafrost to the north and discontinuous permafrost to the south, it is an important place to study permafrost dynamics. Dr. Burke Minsley, geophysicist in the USGS Crustal Geophysics and Geochemistry Science Center in Denver and lead author of the study in Geophysical Research Letters, and his team surveyed more than 116 square miles centered 140 miles northeast of Fairbanks. Their data not only capture in detail the distribution of permafrost and its relation to surface- and groundwater features, but also the legacy of the Yukon River lateral migration over a period of roughly 1,000 years as manifested as a thawed region of permafrost.

Knowledge of the current permafrost distribution is critical for analyses designed to evaluate hydrologic and ecologic consequences of climate warming. It also provides a baseline for future investigation of the dynamic evolution of permafrost systems.

The extent of permafrost can vary as the climate changes. Today, a considerable area of the Arctic is covered by permafrost. Overlying permafrost is a thin active layer that seasonally thaws during the summer. Plant life can be supported only within the active layer since growth can occur only in soil that is fully thawed for some part of the year. Thickness of the active layer varies by year and location, but is typically 2.0—13 feet thick. In areas of continuous permafrost and harsh winters the depth of the permafrost can be as much as 4,898 feet in the northern Lena and Yana River basins in Siberia. Permafrost can also store carbon, both as peat and as methane.

In addition, the study is important because it presents a methodology for assessing permafrost not only in Alaska but throughout sub-Arctic and Arctic regions. The airborne approach allows periodic monitoring of perennially frozen ground over broad areas as climatic warming decreases the extent of permafrost and accelerates the emission of greenhouse gases from that which is stored in the permafrost.